1. Field of the Invention
The invention relates to a process for treating nitrogen oxide-containing gas substantially having a temperature in the range of 300-560.degree. C., for instance exhaust gas of a diesel engine or the off-gas obtained from gas engines. More particularly, the invention relates to a process wherein nitrogen oxide-containing gas is passed over a zeolite-comprising catalyst system in the presence of a NH-containing compound as reducing agent, in order to obtain a gas containing as few environmentally objectionable or noxious compounds as possible.
Every year, more so-called "Total Energy" plants are installed in greenhouses. The most important source of energy for these plants is natural gas. Such plants are used for heating and for providing electric energy for lighting the greenhouses, for which reason these plants are called heat/power coupling plants. The off-gas of the plant, which has a temperature of up to about 450.degree. C. and which contains a high CO.sub.2 content, is essentially suitable to be utilized for fertilizing purposes if this gas is substantially free of nitrogen oxides.
More in detail, the amount of nitrogen oxides in the off-gas of a conventional gas engine will be of the order of 1500-2000 ppm. In order to limit the emission of NO.sub.x, typically so-called lean-burn natural gas engines are used in the greenhouses as heat/power coupling plant. These engines are operated with an excess of air to limit the NO.sub.x emission. More particularly, in this manner the NO.sub.x concentration in the off-gases can be reduced from about 1500-2000 ppm to 400-600 ppm. On the other hand, the efficiency of the engines is reduced by the use of an excess of air. Because the off-gases are used for CO.sub.2 fertilization in closed spaces, the absolute NO.sub.x concentration in these off-gases must meet high standards. The standard applying in this connection is a maximum of 25-50 ppm NO.sub.x. Meanwhile, different catalyst systems have been described which enable the effectuation of NO.sub.x conversions, in such a manner that the NO.sub.x concentration in test systems satisfies the standard mentioned.
The gas engines, however, only deliver a seemingly constant NO.sub.x signal. FIG. 1 shows the amount of NO which is emitted in time by a particular gas engine. At an average NO.sub.x emission level of about 1100 ppm, the signal is found to exhibit a fluctuation of an amplitude of some 100 ppm. This means that in the case or the stationary operation of a process in which NO.sub.x is reduced--which means that a constant stream of the NH-containing compounds is supplied to the off-gas--the standard of 25-50 ppm can only be achieved in the case where a highly complicated and costly control mechanism is employed. A second option, which involves working with an excess of the NH-compound containing reducing agent in order to ensure that a largest possible amount of NO.sub.x is converted is not attractive, since in that case NH-containing compounds slip through along with the treated off-gas. NH compounds are even more noxious than NO.sub.x compounds.
A similar problem presents itself in the treatment of exhaust or off-gases of, for instance, non-stationary diesel engines or other engines of equipments which are operated transiently. Such an engine will be operated highly dynamically, i.e., over time, depending on the load, the NO.sub.x concentration, the gas stream and the temperature of the exhaust gas fluctuate strongly. FIG. 2 gives an impression of these fluctuations. This figure shows a driving cycle or a truck. Plotted along the axes are couple and speed, which parameters can be translated by the skilled person to the gas stream and NO.sub.x concentration.
It will be clear that all this gives problems in regulating the amount of reducing agent which is to be supplied to the catalyst system. These problems could be overcome to a considerable extent if an excess of reducing agent can be used without environmental problems.
Accordingly, there is a need for a process for treating nitrogen oxide-containing gas, wherein nitrogen oxides are reduced in the presence of NH-containing compounds, without the occurrence of any emission (slip) of NH-containing compounds to the environment when these reducing agents are present in excess. More generally, slip of NH compounds is the problem with all known SCR (selective catalytic reduction) processes.
The present invention is directed to an aftertreatment process wherein use is made of a selective catalytic reduction in which a NH-containing compound or composition, for instance ammonia water or an aqueous urea solution, is used as reducing agent.
2. Description of the Related Art
European patent application 0 263 399 describes a method for the catalytic reduction of nitrogen oxides from off-gases coming from power plants and chemical plants. This method relates to a low-temperature deNO.sub.x process, since the nitrogen oxide reduction takes place in the range of 150-350.degree. C. and preferably of 150-280.degree. C. The catalyst which is used in this known method is of the zeolite-Y-type, and in particular of the ion-exchanged zeolite-Y-type. This catalyst is obtained by exchanging ions of sodium zeolite-Y for lithium and/or rare earth ions. One of the possibilities, which, for that matter, is not illustrated in detail, comprises the exchange with Ce ions. Any advantage associated with the use of specifically a CeY zeolite is neither mentioned nor suggested.